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Jim Grant Tech Tips Jim Grant Tech Tips
A collection of helpful articles based on reader submissions
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  Jim Grant's Tech TipsJim Grant's Tech Tips
Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
If you have any questions, please feel free to email us.



Lincoln - Mercury
  Jim Grant's Tech Tips

'98 VW Jetta, Jumpy Temperature Gauge

Q: My ‘98 VW Jetta TDI runs great...but lately (153k) I have noticed the temp gauge has been "jumping around" often moving from the normal operating temperature to very high and then back to normal. I have checked the fans and they seem to be operating correctly and it does not boil over. Can this be a bad thermostat or indicator? If so where is the signal being generated for the temp reading on the dash? Or do I have another problem?

- Tom L.

A: When the temperature gauge is jumping is this at low speed in town driving or the high speed of highway driving? The other question would be is just how fast is the gauge moving/jumping? If a gauge is moving quickly, with little to no rhyme or reason to vehicle speed or load, then I would be inclined to believe that the problem is being caused by a faulty temperature sensor or sensor connection. If the gauge is moving with reason and seems to demonstrate a purpose of movement then there may be a thermostat or cooling system problem. I would also suggest performing a visual inspection on the coolant level. We have found that the coolant level sensor is not always the most reliable beast for that type of simple information. Also, you should know that the coolant temperature is easily monitored through the diagnostic connector for the vehicle computer. Just plugging in a scan tool can provide as close to real live coolant temperature information that can be had. A test drive with a computer scan tool should answer a lot of questions.

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